Dominion to me "seems" to be faster ip per time spent ingame but don't count on it. It's likely that they both use the same ip algorithm to work out how much you get for the amount of time you spent playing and whether you won or lost.

That depends, you can win in 10 minutes in Dominion but my last game was 25 minutes long (we were losing like 450 to 100 and won ! that was epic ). Game in Classic can take 20 minutes (surrender) but my longest match was 1 hour and 10 minutes long. but if you will calculate IP/hour I think it will be pretty much the same numbers (or Dominion bit higher thanks to additional bonus).

You can calculate it, but it requires some work. You need three things from games:
1. How much IP did you gain?
2. Did you win or lose?
3. What was the game length in minutes and seconds?

You can get this data from post-game screen only. When we get lots of these reports we can plot them in scale and get average time for games. That's the only way to get actually accurate IP gains per hour.